Lung Analysis Advised Prior to Ventilation for ARDS

Action Points

Explain to interested patients that this study suggests that when considering positive end-expiratory pressure (PEEP) for patients with acute respiratory distress syndrome (ARDS), the percentage of potentially recruitable lung was associated with the response to treatment.

Recognize that although PEEP is routinely used in the ventilator management of ARDS, there is no consensus on how to choose an optimal level of PEEP to improve survival.

The Italian investigators found that the amount of potentially recruitable lung regions vary greatly and are strongly associated with the response to positive end-expiratory pressure (PEEP).

The percentage of collapsed lung units that could be opened, as identified by CT scans, varied from a negligible fraction to more than 50% of total lung weight and was negatively correlated with the response to PEEP (r2=0.72m P<0.001), found a study in the April 27 New England Journal of Medicine.

In the study of 68 patients with acute lung injury or ARDS who underwent whole-lung CT during breath-holding sessions at airway pressures of 5, 15, and 45 cm of water, 24% of the lung on average could not be recruited, even at an airway pressure of 45 cm water.

Patients with more than the median value of 9% of recruitable lung had greater lung weights (P<0.001), poorer oxygenation (P<0.001) and respiratory-system compliance (P=0.002), higher levels of dead space (P=0.002), and higher death rates (P=.02) compared with patients having a lower percentage of potentially recruitable lung, the researchers reported.

The combined physiological variables predicted, with a sensitivity of 71% and a specificity of 59%, whether a patient's proportion of potentially recruitable lung was higher or lower than the median, the researchers said.

The average of potentially recruitable lung, as assessed by CT, was 13Â±11% of the total lung weight (95% CI 10-16%; median 9%), corresponding to an absolute weight of 217Â±232 g of recruitable lung tissue (95% CI, 161-273), the researchers reported.

An examination of the association between the recruitable lung and the severity of overall lung injury found that in those with more recruitable lung compared with those with less, total lung weight was greater, the proportion of nonaerated lung tissue was higher, the respiratory system compliance was lower, the PaCo2 was higher, the percentage of dead space was higher, and the mortality rate was higher.

The overall mortality rate in the ICU was 28%, the researchers said. Noting the association between the percentage of potentially recruitable lung and the risk of death, the researchers did a multivariate analysis of independent predictors for mortality.

Using the SAPS II score (Simplified Acute Physiology Score), they found that the percentage of recruitable lung appeared to be independently associated with an increased risk of death (P=0.47). The odds ratio for each one-point increase in the SAPS II score and the percentage of potentially recruitable lung were 1.08 (95% CI, 10.2 to 1.15) and 1.05 (95% CI, 1.01-1.14) respectively, they said.

Setting levels of PEEP without knowing the percentage of potentially recruitable lung may offset the possible benefits of PEEP and may actually be harmful, Dr. Gattinoni said.

A formal study is needed, he suggested, to determine how different PEEP levels may affect these patients, but it should be limited to patients with a higher percentage of potentially recruitable lung.

In conclusion, he said, "While we wait of such a study to be performed, in our daily practice we limit the use of PEEP levels of more than 15 cm of water to patients with a higher percentage of potentially recruitable lung and of PEEP levels below 10 cm of water to those with a lower percentage of potentially recruitable lung."

In an accompanying editorial, Arthur Slutsky, M.D., at the University of Toronto, and Leonard Hudson, M.D., at the University of Washington in Seattle, wrote that the Italian study provides a potential solution to the problem of identifying which patients may benefit from PEEP and which patients may potentially be harmed.

In reviewing the study, they mentioned several caveats:

A number of patients do not respond to pressures of 45 cm of water but would respond at higher pressures.

The researchers did not address the issue of how much lung tissue is sufficient to consider use of higher levels of PEEP in a given patient.

The study could not determine which level of PEEP should be used even for patients with a large amount of recruitable lung.

The use of CT is not practical is most clinical settings.

A major message of this study, they concluded, is that future studies of strategies for PEEP levels must take into account the degree to which the lungs can be recruited. In this postgenomic era, they wrote, Dr. Gattinoni and colleagues "demonstrate that sound physiological principles are still relevant to our understanding of disease processes."

Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco

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